chapter 7: relational database designdatabase system concepts - 7th edition 6.7 ©silberschatz,...

Database System Concepts, 7th Ed.

©Silberschatz, Korth and Sudarshan

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Chapter 6: Database Design Using the E-R

Model

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©Silberschatz, Korth and Sudarshan6.4Database System Concepts - 7th Edition

Design Phases

▪ Initial phase -- characterize fully the data needs of the prospective

database users.

▪ Second phase -- choosing a data model

• Applying the concepts of the chosen data model

• Translating these requirements into a conceptual schema of the

database.

• A fully developed conceptual schema indicates the functional

requirements of the enterprise.

▪ Describe the kinds of operations (or transactions) that will be

performed on the data.


Design Phases (Cont.)

▪ Final Phase -- Moving from an abstract data model to the implementation

of the database

• Logical Design – Deciding on the database schema.

▪ Database design requires that we find a “good” collection of

relation schemas.

▪ Business decision – What attributes should we record in the

database?

▪ Computer Science decision – What relation schemas should we

have and how should the attributes be distributed among the

various relation schemas?

• Physical Design – Deciding on the physical layout of the database


Design Alternatives

▪ In designing a database schema, we must ensure that we avoid two

major pitfalls:

• Redundancy: a bad design may result in repeat information.

▪ Redundant representation of information may lead to data

inconsistency among the various copies of information

• Incompleteness: a bad design may make certain aspects of the

enterprise difficult or impossible to model.

▪ Avoiding bad designs is not enough. There may be a large number of

good designs from which we must choose.


Design Approaches

▪ Entity Relationship Model (covered in this chapter)

• Models an enterprise as a collection of entities and relationships

▪ Entity: a “thing” or “object” in the enterprise that is distinguishable

from other objects

• Described by a set of attributes

▪ Relationship: an association among several entities

• Represented diagrammatically by an entity-relationship diagram:

▪ Normalization Theory (Chapter 7)

• Formalize what designs are bad, and test for them


Outline of the ER Model


Entity Sets

▪ An entity is an object that exists and is distinguishable from other

objects.

• Example: specific person, company, event, plant

▪ An entity set is a set of entities of the same type that share the same

properties.

• Example: set of all persons, companies, trees, holidays

▪ An entity is represented by a set of attributes; i.e., descriptive properties

possessed by all members of an entity set.

• Example:

instructor = (ID, name, salary )

course= (course_id, title, credits)

▪ A subset of the attributes form a primary key of the entity set; i.e.,

uniquely identifying each member of the set.


Representing Entity sets in ER Diagram

▪ Entity sets can be represented graphically as follows:

• Rectangles represent entity sets.

• Attributes listed inside entity rectangle

• Underline indicates primary key attributes


Relationship Sets

▪ A relationship is an association among several entities

Example:

44553 (Peltier) advisor 22222 (Einstein)

student entity relationship set instructor entity

▪ A relationship set is a mathematical relation among n 2 entities, each

taken from entity sets

{(e1, e2, … en) | e1 E1, e2 E2, …, en En}

where (e1, e2, …, en) is a relationship

• Example:

(44553,22222) advisor


Relationship Sets (Cont.)

▪ Example: we define the relationship set advisor to denote the

associations between students and the instructors who act as their

advisors.

▪ Pictorially, we draw a line between related entities.


Representing Relationship Sets via ER Diagrams

▪ Diamonds represent relationship sets.


Relationship Sets (Cont.)

▪ An attribute can also be associated with a relationship set.

▪ For instance, the advisor relationship set between entity sets instructor

and student may have the attribute date which tracks when the student

started being associated with the advisor

instructor

student

76766 Crick

Katz

Srinivasan

Kim

Singh

Einstein

45565

10101

98345

76543

22222

98988

12345

00128

76543

44553

Tanaka

Shankar

Zhang

Brown

Aoi

Chavez

Peltier

3 May 2008

10 June 2007

12 June 2006

6 June 2009

30 June 2007

31 May 2007

4 May 2006

76653

23121


Relationship Sets with Attributes


Roles

▪ Entity sets of a relationship need not be distinct

• Each occurrence of an entity set plays a “role” in the relationship

▪ The labels “course_id” and “prereq_id” are called roles.


Degree of a Relationship Set

▪ Binary relationship

• involve two entity sets (or degree two).

• most relationship sets in a database system are binary.

▪ Relationships between more than two entity sets are rare. Most

relationships are binary. (More on this later.)

• Example: students work on research projects under the guidance of

an instructor.

• relationship proj_guide is a ternary relationship between instructor,

student, and project


Non-binary Relationship Sets

▪ Most relationship sets are binary

▪ There are occasions when it is more convenient to represent

relationships as non-binary.

▪ E-R Diagram with a Ternary Relationship


Complex Attributes

▪ Attribute types:

• Simple and composite attributes.

• Single-valued and multivalued attributes

▪ Example: multivalued attribute: phone_numbers

• Derived attributes

▪ Can be computed from other attributes

▪ Example: age, given date_of_birth

▪ Domain – the set of permitted values for each attribute


Composite Attributes

▪ Composite attributes allow us to divided attributes into subparts (other

attributes).

name address

first_name middle_initial last_name street city state postal_code

street_number street_name apartment_number

compositeattributes

componentattributes


Representing Complex Attributes in ER Diagram


Mapping Cardinality Constraints

▪ Express the number of entities to which another entity can be associated

via a relationship set.

▪ Most useful in describing binary relationship sets.

▪ For a binary relationship set the mapping cardinality must be one of the

following types:

• One to one

• One to many

• Many to one

• Many to many


Mapping Cardinalities

One to one One to many

Note: Some elements in A and B may not be mapped to any

elements in the other set


Mapping Cardinalities

Many to one Many to many

Note: Some elements in A and B may not be mapped to any

elements in the other set


Representing Cardinality Constraints in ER Diagram

▪ We express cardinality constraints by drawing either a directed line (→), signifying “one,” or an undirected line (—), signifying “many,” between the relationship set and the entity set.

▪ One-to-one relationship between an instructor and a student :

• A student is associated with at most one instructor via the relationship advisor

• A student is associated with at most one department via stud_dept


One-to-Many Relationship

▪ one-to-many relationship between an instructor and a student

• an instructor is associated with several (including 0) students via

advisor

• a student is associated with at most one instructor via advisor,


Many-to-One Relationships

▪ In a many-to-one relationship between an instructor and a student,

• an instructor is associated with at most one student via advisor,

• and a student is associated with several (including 0) instructors via

advisor


Many-to-Many Relationship

▪ An instructor is associated with several (possibly 0) students via advisor

▪ A student is associated with several (possibly 0) instructors via advisor


Total and Partial Participation

▪ Total participation (indicated by double line): every entity in the entity set

participates in at least one relationship in the relationship set

participation of student in advisor relation is total

▪ every student must have an associated instructor

▪ Partial participation: some entities may not participate in any relationship

in the relationship set

• Example: participation of instructor in advisor is partial


Notation for Expressing More Complex Constraints

▪ A line may have an associated minimum and maximum cardinality, shown

in the form l..h, where l is the minimum and h the maximum cardinality

• A minimum value of 1 indicates total participation.

• A maximum value of 1 indicates that the entity participates in at most

one relationship

• A maximum value of * indicates no limit.

▪ Example

• Instructor can advise 0 or more students. A student must have 1

advisor; cannot have multiple advisors


Cardinality Constraints on Ternary Relationship

▪ We allow at most one arrow out of a ternary (or greater degree)

relationship to indicate a cardinality constraint

▪ For example, an arrow from proj_guide to instructor indicates each

student has at most one guide for a project

▪ If there is more than one arrow, there are two ways of defining the

meaning.

• For example, a ternary relationship R between A, B and C with

arrows to B and C could mean

1. Each A entity is associated with a unique entity from B

and C or

2. Each pair of entities from (A, B) is associated with a

unique C entity, and each pair (A, C) is associated

with a unique B

• Each alternative has been used in different formalisms

• To avoid confusion we outlaw more than one arrow


Primary Key

▪ Primary keys provide a way to specify how entities and relations are

distinguished. We will consider:

• Entity sets

• Relationship sets.

• Weak entity sets


Primary key for Entity Sets

▪ By definition, individual entities are distinct.

▪ From database perspective, the differences among them must be

expressed in terms of their attributes.

▪ The values of the attribute values of an entity must be such that they can

uniquely identify the entity.

• No two entities in an entity set are allowed to have exactly the same

value for all attributes.

▪ A key for an entity is a set of attributes that suffice to distinguish entities

from each other


Primary Key for Relationship Sets

▪ To distinguish among the various relationships of a relationship set we use

the individual primary keys of the entities in the relationship set.

• Let R be a relationship set involving entity sets E1, E2, .. En

• The primary key for R is consists of the union of the primary keys of

entity sets E1, E2, ..En

• If the relationship set R has attributes a1, a2, .., am associated with it,

then the primary key of R also includes the attributes a1, a2, .., am

▪ Example: relationship set “advisor”.

• The primary key consists of instructor.ID and student.ID

▪ The choice of the primary key for a relationship set depends on the

mapping cardinality of the relationship set.


Choice of Primary key for Binary Relationship

▪ Many-to-Many relationships. The preceding union of the primary keys is a

minimal superkey and is chosen as the primary key.

▪ One-to-Many relationships . The primary key of the “Many” side is a

minimal superkey and is used as the primary key.

▪ Many-to-one relationships. The primary key of the “Many” side is a minimal

superkey and is used as the primary key.

▪ One-to-one relationships. The primary key of either one of the participating

entity sets forms a minimal superkey, and either one can be chosen as the

primary key.


Weak Entity Sets

▪ Consider a section entity, which is uniquely identified by a course_id,

semester, year, and sec_id.

▪ Clearly, section entities are related to course entities. Suppose we create

a relationship set sec_course between entity sets section and course.

▪ Note that the information in sec_course is redundant, since section

already has an attribute course_id, which identifies the course with which

the section is related.

▪ One option to deal with this redundancy is to get rid of the relationship

sec_course; however, by doing so the relationship between section and

course becomes implicit in an attribute, which is not desirable.


Weak Entity Sets (Cont.)

▪ An alternative way to deal with this redundancy is to not store the attribute

course_id in the section entity and to only store the remaining attributes

section_id, year, and semester.

• However, the entity set section then does not have enough attributes

to identify a particular section entity uniquely

▪ To deal with this problem, we treat the relationship sec_course as a

special relationship that provides extra information, in this case, the

course_id, required to identify section entities uniquely.

▪ A weak entity set is one whose existence is dependent on another entity,

called its identifying entity

▪ Instead of associating a primary key with a weak entity, we use the

identifying entity, along with extra attributes called discriminator to

uniquely identify a weak entity.


Weak Entity Sets (Cont.)

▪ An entity set that is not a weak entity set is termed a strong entity set.

▪ Every weak entity must be associated with an identifying entity; that is,

the weak entity set is said to be existence dependent on the identifying

entity set.

▪ The identifying entity set is said to own the weak entity set that it

identifies.

▪ The relationship associating the weak entity set with the identifying entity

set is called the identifying relationship.

▪ Note that the relational schema we eventually create from the entity set

section does have the attribute course_id, for reasons that will become

clear later, even though we have dropped the attribute course_id from

the entity set section.


Expressing Weak Entity Sets

▪ In E-R diagrams, a weak entity set is depicted via a double rectangle.

▪ We underline the discriminator of a weak entity set with a dashed line.

▪ The relationship set connecting the weak entity set to the identifying

strong entity set is depicted by a double diamond.

▪ Primary key for section – (course_id, sec_id, semester, year)


Redundant Attributes

▪ Suppose we have entity sets:

• student, with attributes: ID, name, tot_cred, dept_name

• department, with attributes: dept_name, building, budget

▪ We model the fact that each student has an associated department using

a relationship set stud_dept

▪ The attribute dept_name in student below replicates information present

in the relationship and is therefore redundant

• and needs to be removed.

▪ BUT: when converting back to tables, in some cases the attribute gets

reintroduced, as we will see later.


E-R Diagram for a University Enterprise


Reduction to Relation Schemas


Reduction to Relation Schemas

▪ Entity sets and relationship sets can be expressed uniformly as relation

schemas that represent the contents of the database.

▪ A database which conforms to an E-R diagram can be represented by a

collection of schemas.

▪ For each entity set and relationship set there is a unique schema that is

assigned the name of the corresponding entity set or relationship set.

▪ Each schema has a number of columns (generally corresponding to

attributes), which have unique names.


Representing Entity Sets

▪ A strong entity set reduces to a schema with the same attributes

student(ID, name, tot_cred)

▪ A weak entity set becomes a table that includes a column for the primary

key of the identifying strong entity set

section ( course_id, sec_id, sem, year )

▪ Example


Representation of Entity Sets with Composite Attributes

▪ Composite attributes are flattened out by creating a

separate attribute for each component attribute

• Example: given entity set instructor with composite

attribute name with component attributes first_name

and last_name the schema corresponding to the

entity set has two attributes name_first_name and

name_last_name

▪ Prefix omitted if there is no ambiguity

(name_first_name could be first_name)

▪ Ignoring multivalued attributes, extended instructor

schema is

• instructor(ID,

first_name, middle_initial, last_name,

street_number, street_name,

apt_number, city, state, zip_code,

date_of_birth)


Representation of Entity Sets with Multivalued Attributes

▪ A multivalued attribute M of an entity E is represented by a separate

schema EM

▪ Schema EM has attributes corresponding to the primary key of E and an

attribute corresponding to multivalued attribute M

▪ Example: Multivalued attribute phone_number of instructor is

represented by a schema:

inst_phone= ( ID, phone_number)

▪ Each value of the multivalued attribute maps to a separate tuple of the

relation on schema EM

• For example, an instructor entity with primary key 22222 and phone

numbers 456-7890 and 123-4567 maps to two tuples:

(22222, 456-7890) and (22222, 123-4567)


Representing Relationship Sets

▪ A many-to-many relationship set is represented as a schema with

attributes for the primary keys of the two participating entity sets, and

any descriptive attributes of the relationship set.

▪ Example: schema for relationship set advisor

advisor = (s_id, i_id)


Redundancy of Schemas

▪ Many-to-one and one-to-many relationship sets that are total on the many-side can be represented by adding an extra attribute to the “many” side, containing the primary key of the “one” side

▪ Example: Instead of creating a schema for relationship set inst_dept, add an attribute dept_name to the schema arising from entity set instructor

▪ Example


Redundancy of Schemas (Cont.)

▪ For one-to-one relationship sets, either side can be chosen to act as the “many” side

• That is, an extra attribute can be added to either of the tables corresponding to the two entity sets

▪ If participation is partial on the “many” side, replacing a schema by an extra attribute in the schema corresponding to the “many” side could result in null values


Redundancy of Schemas (Cont.)

▪ The schema corresponding to a relationship set linking a weak entity set to its identifying strong entity set is redundant.

▪ Example: The section schema already contains the attributes that would appear in the sec_course schema


Extended E-R Features


Specialization

▪ Top-down design process; we designate sub-groupings within an entity set

that are distinctive from other entities in the set.

▪ These sub-groupings become lower-level entity sets that have attributes or

participate in relationships that do not apply to the higher-level entity set.

▪ Depicted by a triangle component labeled ISA (e.g., instructor “is a”

person).

▪ Attribute inheritance – a lower-level entity set inherits all the attributes

and relationship participation of the higher-level entity set to which it is

linked.


Specialization Example

▪ Overlapping – employee and student

▪ Disjoint – instructor and secretary

▪ Total and partial


Representing Specialization via Schemas

▪ Method 1:

• Form a schema for the higher-level entity

• Form a schema for each lower-level entity set, include primary key

of higher-level entity set and local attributes

• Drawback: getting information about, an employee requires

accessing two relations, the one corresponding to the low-level

schema and the one corresponding to the high-level schema


Representing Specialization as Schemas (Cont.)

▪ Method 2:

• Form a schema for each entity set with all local and inherited

attributes

• Drawback: name, street and city may be stored redundantly for

people who are both students and employees


Generalization

▪ A bottom-up design process – combine a number of entity sets that

share the same features into a higher-level entity set.

▪ Specialization and generalization are simple inversions of each other;

they are represented in an E-R diagram in the same way.

▪ The terms specialization and generalization are used interchangeably.


Completeness constraint

▪ Completeness constraint -- specifies whether or not an entity in the

higher-level entity set must belong to at least one of the lower-level

entity sets within a generalization.

• total: an entity must belong to one of the lower-level entity sets

• partial: an entity need not belong to one of the lower-level entity

sets


Completeness constraint (Cont.)

▪ Partial generalization is the default.

▪ We can specify total generalization in an ER diagram by adding the

keyword total in the diagram and drawing a dashed line from the

keyword to the corresponding hollow arrow-head to which it applies (for

a total generalization), or to the set of hollow arrow-heads to which it

applies (for an overlapping generalization).

▪ The student generalization is total: All student entities must be either

graduate or undergraduate. Because the higher-level entity set arrived

at through generalization is generally composed of only those entities

in the lower-level entity sets, the completeness constraint for a

generalized higher-level entity set is usually total


Aggregation

▪ Consider the ternary relationship proj_guide, which we saw earlier

▪ Suppose we want to record evaluations of a student by a guide on a

project


Aggregation (Cont.)

▪ Relationship sets eval_for and proj_guide represent overlapping

information

• Every eval_for relationship corresponds to a proj_guide relationship

• However, some proj_guide relationships may not correspond to any

eval_for relationships

▪ So we can’t discard the proj_guide relationship

▪ Eliminate this redundancy via aggregation

• Treat relationship as an abstract entity

• Allows relationships between relationships

• Abstraction of relationship into new entity


Aggregation (Cont.)

▪ Eliminate this redundancy via aggregation without introducing

redundancy, the following diagram represents:

• A student is guided by a particular instructor on a particular project

• A student, instructor, project combination may have an associated

evaluation


Reduction to Relational Schemas

▪ To represent aggregation, create a schema containing

• Primary key of the aggregated relationship,

• The primary key of the associated entity set

• Any descriptive attributes

▪ In our example:

• The schema eval_for is:

eval_for (s_ID, project_id, i_ID, evaluation_id)

• The schema proj_guide is redundant.


Design Issues


Common Mistakes in E-R Diagrams

▪ Example of erroneous E-R diagrams


Common Mistakes in E-R Diagrams (Cont.)


Entities vs. Attributes

▪ Use of entity sets vs. attributes

▪ Use of phone as an entity allows extra information about phone numbers

(plus multiple phone numbers)


Entities vs. Relationship sets

▪ Use of entity sets vs. relationship sets

Possible guideline is to designate a relationship set to describe

an action that occurs between entities

▪ Placement of relationship attributes

For example, attribute date as attribute of advisor or as attribute

of student


Binary Vs. Non-Binary Relationships

▪ Although it is possible to replace any non-binary (n-ary, for n > 2)

relationship set by a number of distinct binary relationship sets, a n-ary

relationship set shows more clearly that several entities participate in a

single relationship.

▪ Some relationships that appear to be non-binary may be better

represented using binary relationships

• For example, a ternary relationship parents, relating a child to

his/her father and mother, is best replaced by two binary

relationships, father and mother

▪ Using two binary relationships allows partial information (e.g.,

only mother being known)

• But there are some relationships that are naturally non-binary

▪ Example: proj_guide


Converting Non-Binary Relationships to Binary Form

▪ In general, any non-binary relationship can be represented using binary relationships by creating an artificial entity set.

• Replace R between entity sets A, B and C by an entity set E, and three relationship sets:

1. RA, relating E and A 2. RB, relating E and B 3. RC, relating E and C

• Create an identifying attribute for E and add any attributes of R to E

• For each relationship (ai , bi , ci) in R, create

1. a new entity ei in the entity set E 2. add (ei , ai ) to RA

3. add (ei , bi ) to RB 4. add (ei , ci ) to RC


Converting Non-Binary Relationships (Cont.)

▪ Also need to translate constraints

• Translating all constraints may not be possible

• There may be instances in the translated schema that

cannot correspond to any instance of R

▪ Exercise: add constraints to the relationships RA, RB and RC to

ensure that a newly created entity corresponds to exactly one

entity in each of entity sets A, B and C

• We can avoid creating an identifying attribute by making E a weak

entity set (described shortly) identified by the three relationship sets


E-R Design Decisions

▪ The use of an attribute or entity set to represent an object.

▪ Whether a real-world concept is best expressed by an entity set or a

relationship set.

▪ The use of a ternary relationship versus a pair of binary relationships.

▪ The use of a strong or weak entity set.

▪ The use of specialization/generalization – contributes to modularity in the

design.

▪ The use of aggregation – can treat the aggregate entity set as a single

unit without concern for the details of its internal structure.


Summary of Symbols Used in E-R Notation


Symbols Used in E-R Notation (Cont.)


Alternative ER Notations

▪ Chen, IDE1FX, …


Alternative ER Notations

Chen IDE1FX (Crows feet notation)


UML

▪ UML: Unified Modeling Language

▪ UML has many components to graphically model different aspects of an

entire software system

▪ UML Class Diagrams correspond to E-R Diagram, but several

differences.


ER vs. UML Class Diagrams

* Note reversal of position in cardinality constraint depiction



ER Diagram Notation Equivalent in UML

* Generalization can use merged or separate arrows independent

of disjoint/overlapping


UML Class Diagrams (Cont.)

▪ Binary relationship sets are represented in UML by just drawing a line

connecting the entity sets. The relationship set name is written adjacent

to the line.

▪ The role played by an entity set in a relationship set may also be

specified by writing the role name on the line, adjacent to the entity set.

▪ The relationship set name may alternatively be written in a box, along

with attributes of the relationship set, and the box is connected, using a

dotted line, to the line depicting the relationship set.


Other Aspects of Database Design

▪ Functional Requirements

▪ Data Flow, Workflow

▪ Schema Evolution


End of Chapter 6

chapter 7: relational database designdatabase system concepts - 7th edition 6.7 ©silberschatz,...

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